One suggestion that I would make is to be careful to be inclusive, and not allow the focus of the group to wander.
The atheist group that I've attended has been taken over by radical political liberals, and has gotten essentially turned into a liberal club, with a political litmus test required for membership. Anyone who disagrees with their extreme political agenda is unwelcome; atheist or no.
So my advice is to emphasize your commonalities, and discourage cliques based on differences. Things like that can kill a group quickly.
I am a member of a newly formed Humanist group. We have about 30 members and are a member of the American Humanist Association. I would say about half of our somewhat small group are members of my UU church. Most of our meetings are at my UU church. We are just starting out like your group and we are still sort of defining our purpose and goals. But being a member of a national groups helps. Did you consider joining the American Humanist Association or a similar secular group?
I should mention that the guy who started this UU group on Atheist Nexus is the head of our Humanist Group, South Jersey Humanist.
I hope your group is going well. There certainly are others. Did you find the UU Humanist group?